A light emitting diode (LED) is a compound semiconductor having p-n junction structure of semiconductor and refers to a device for emitting predetermined light through recombination of minority carriers (electrons or holes). LEDs include a red LED using GaAsP or the like, a green LED using GaP or the like, a blue LED using an InGaN/AlGaN double hetero structure, and the like.
A light emitting device using the LED has characteristics of low power consumption, a long lifespan, installation in a narrow space, and strong resistance against vibration. In recent years, white light emitting devices in addition to single color light emitting devices, e.g. red, blue or green light emitting devices, have been placed on the market. As the white light emitting devices are applied to products for automobiles and illumination, it is expected that their demands will be rapidly increased.
In the light emitting device technologies, the methods of implementing white color can be roughly classified into two types. The first one is a method in which red, blue and green LED chips are arranged to be adjacent to one another and colors of light emitted from the respective LEDs are mixed to implement white color. However, since the respective LED chips have different thermal or temporal characteristics, there are problems in that uniform color mixing cannot be obtained due to changes in a color tone according to usage environment, particularly, the occurrence of color spots, or the like, and thus, the brightness is not sufficiently high. Further, the circuit configurations for operating the respective LED chips are complex, and it is difficult to implement perfect white light since it is difficult to obtain optimal conditions for mixing three color lights depending on the positions of the chips. Moreover, since its color rendering index is as low as about 40, there is a problem in that it is not suitable for the general lighting source or the flash.
The second one is a method in which a phosphor is disposed on an LED chip and the color of a portion of primary light emitted from the LED chip and the color of secondary light of which wavelength has been converted by the phosphor are mixed to implement white color. For example, onto an LED chip for emitting blue light is attached a phosphor that emits yellowish green or yellow using a portion of the blue light as an excitation source, so that white light can be obtained by mixing the blue light emitted from the light emitting diode chip and the yellowish green or yellow light emitted from the phosphor. However, there is a problem in that such a light emitting device corresponds to light emission from a single yellow phosphor and has a low color rendering index due to spectrum shortage of green and red colors.
In order to solve the problem related to the low color rendering index as described above, a blue LED chip and green and red phosphors excited by the blue light of the blue LED chip are used to manufacture a white light emitting device. That is, the white light with a high color rendering index of 85 or more can be implemented by mixing blue light with green and red light emitted from the phosphors excited by the blue light. At this time, a thiogallate based phosphor excited by blue light to emit green to yellow light is used as a green light emitting phosphor. The typical thiogallate based phosphor has a composition expressed as (Ca,Sr,Ba)(Al,Ga,In)2S4:Eu(or Ce). Among the composition, a SrGa2S4:Eu phosphor is a green light emitting phosphor with high luminous intensity. Meanwhile, SrS:Eu, (Sr,Ca)S:Eu, CaS:Eu and the like are used as a red light emitting phosphor excited by the blue light to emit red light.
However, since the red phosphor has very poor luminous characteristics, a red phosphor with excellent luminous efficiency needs be developed. Specifically, the red light emitting phosphor expressed as (Sr,Ca)S:Eu has low luminous intensity and poor chemical stability against moisture. As a result, the application of the red phosphor to a general lighting source and LCD backlight lighting device is restricted.
The present invention is conceived to solve the aforementioned problems in the prior art. Accordingly, an object of the present invention is to provide a red phosphor with improved reliability and luminous efficiency and a method for manufacturing the same. Another object of the present invention is to provide a light emitting device with more excellent color reproducibility and optical characteristics using the red phosphor with improved reliability and luminous efficiency.
According to an aspect of the present invention for achieving the objects, there is provided a red phosphor having a chemical formula of (Ca,Sr)1-x-yEuxPbyS, wherein 0.0005≦x≦0.01 and 0.001≦y≦0.05. Preferably, a range of x is 0.001≦x≦0.005 and a range of y is 0.005≦y≦0.03.
The red phosphor may emit light with a wavelength of 600 nm to 660 nm. According to another aspect of the present invention, there is provided a method for manufacturing a red phosphor, comprising the steps of quantifying starting materials containing Sr, Ca, Eu and Pb, respectively, and dissolving the starting materials in nitric acid or hydrochloric acid; adding (NH4)2CO3 or (NH4)2C2O4 to precipitate the solution in a state where the respective starting materials are dispersed; and drying the precipitate and then heat-treating the dried precipitate at a temperature of 900 to 1250° C. for 1 to 5 hours under a sulfur dioxide atmosphere. Preferably, the starting material is selected from the group consisting of SrCO3, CaCO3, Eu2O3 and Pb(NO3)2.
According to a further aspect of the present invention, there is provided a light emitting device, comprising a light emitting diode chip; and a red phosphor having a chemical formula of (Ca,Sr)1-x-yEuxPbyS, wherein 0.0005≦x≦0.01 and 0.001≦y≦0.05. The light emitting diode chip may emit blue light or ultraviolet rays. Preferably, the light emitting diode chip emits blue light and the light emitting device further includes a green light emitting phosphor. Preferably, the green light emitting phosphor is (Ba,Sr,Ca)2SiO4:Eu or SrGa2S4:Eu.
Preferably, the light emitting diode chip is mounted on a body, a molding portion for sealing the light emitting diode chip is formed over the body, and the phosphor is distributed in the molding portion. The body may be any one of a substrate, a heat sink and a lead terminal.